Re: arm_cpu_post_init (Was: Re: arm: "max" CPU class hierarchy changes possible?)

[Claudio Fontana]

On 3/18/21 12:06 PM, Claudio Fontana wrote:
> On 3/11/21 8:10 PM, Andrew Jones wrote:
>> On Thu, Mar 11, 2021 at 06:33:15PM +0000, Peter Maydell wrote:
>>> On Thu, 11 Mar 2021 at 17:16, Claudio Fontana <cfontana@suse.de> wrote:
>>>> Maybe Peter you could clarify similarly what the intended meaning of "max" 
>>>> is on ARM?
>>>
>>> "max" is "best we can do, whatever that is". (On KVM this is "same as
>>> the host".)
>>> "host" is "whatever the host is (KVM only)".
>>>
>>>> KVM: (aarch64-only): aarch64_max_initfn():
>>>>
>>>> The following comment in the code seems wrong to me:
>>>>
>>>> /* -cpu max: if KVM is enabled, like -cpu host (best possible with this 
>>>> host); */
>>>>
>>>> This is not exactly true:
>>>>
>>>> "-cpu max" calls kvm_arm_set_cpu_features_from_host(), (which checks 
>>>> "dtb_compatible", and if not set gets the features from the host, if set 
>>>> ...?)
>>>> After that, calls aarch64_add_sve_properties() and then adds also 
>>>> "svw-max-vq". This code is common with TCG.
> 
> 
> As part of this research I noticed that arm_cpu_post_init() is quite 
> confusing, seems really inconsistent to me.
> 
> Apparently the intention was to call it from the leaf classes:
> 
> commit 51e5ef459eca045d7e8afe880ee60190f0b75b26
> Author: Marc-André Lureau <marcandre.lureau@redhat.com>
> Date:   Tue Nov 27 12:55:59 2018 +0400
> 
>     arm: replace instance_post_init()
>     
>     Replace arm_cpu_post_init() instance callback by calling it from leaf
>     classes, to avoid potential ordering issue with other post_init callbacks.
>     
>     Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
>     Suggested-by: Igor Mammedov <imammedo@redhat.com>
>     Reviewed-by: Igor Mammedov <imammedo@redhat.com>
>     Acked-by: Eduardo Habkost <ehabkost@redhat.com>
> 
> 
> but then we end up calling it multiple times in the class hierarch, which is 
> a recipe for bugs, and makes it difficult to understand what 
> arm_cpu_post_init()
> even means, what calling this function is supposed to do.
> 
> For a "max" or "host" cpu on AArch64, this function is called:
> 
> for the ARM CPU base class, TYPE_ARM_CPU, in
> 
> cpu.c::arm_cpu_instance_init,
> 
> then later again for the TYPE_AARCH64_CPU class, child of TYPE_ARM_CPU, in
> 
> cpu64.c::aarch64_cpu_instance_init,
> 
> then later again for the TYPE_ARM_HOST_CPU class, child of TYPE_AARCH64_CPU, 
> in
> 
> cpu.c::arm_host_initfn.
> 
> Same for "max".
> 
> When looking at 32bit CPUs instead, only the ARM CPU base class ends up 
> calling arm_cpu_post_init.
> "Leaf" classes do not do it (see cpu_tcg.c).
> 
> What is then arm_cpu_post_init even supposed to mean?


And why do we have a separate arm_cpu_finalize_features()?

Nothing in the ARM cpu classes initializations ever seems to be "final" to me.



> 
> Thanks,
> 
> Claudio
> 
> 
>>>>
>>>> In the case of cpu host instead,
>>>>
>>>> "-cpu host" calls kvm_arm_set_cpu_features_from_host(), same as max, then 
>>>> calls aarch64_add_sve_properties() but does NOT add "svw-max-vq".
>>>>
>>>> Is this a bug?
>>
>> It was left out intentionally. More below.
>>
>>>
>>> Maybe; that's a question for Richard or Drew...
>>>
>>>> Are "max" and "host" for KVM supposed to be the same like with x86?
>>
>> Yes, but my understanding of "max" == "host" for KVM is that that only
>> applies to the perspective of the guest. What CPU and what CPU features
>> the guest can see should be exactly the same with either "max" or "host",
>> depending on the enabling/disabling of any optional CPU properties.
>>
>> The question here seems to be that, if one has a CPU property, does that
>> imply the other should have the same? Which would effectively allow the
>> two to be aliases (when KVM is enabled). I don't know, does x86 ensure
>> 100% property compatibility?
>>
>> I opted not to support sve-max-vq for "host" because I consider it a
>> legacy CPU property, one I didn't want to propagate. Indeed it may
>> make more sense to depreciate sve-max-vq than to "fix" this issue
>> by adding it to "host". Note, we can already create equivalent SVE
>> CPUs. The following are the same from the perspective of the guest
>>
>>  -accel kvm -cpu host,sve512=on
>>  -accel kvm -cpu max,sve512=on
>>
>> And, for TCG, these are the same from the perspective of the guest
>>  
>>  -accel tcg -cpu max,sve512=on
>>  -accel tcg -cpu max,sve-max-vq=4
>>
>> So we already don't need sve-max-vq.
>>
>> Thanks,
>> drew
>>
>